What to Put Down Before Tile: Subfloor, Underlayment, and Prep Guide

Preparing the surface before installing tile is critical to a long-lasting, professional result. This guide explains what to put down before tile, covering subfloor types, underlayment options, waterproofing, and installation tips so homeowners and DIYers can make informed choices. Proper preparation prevents cracks, movement, and moisture problems.

Surface	Recommended Layer Before Tile	When To Use
Concrete Slab	Thin-set over clean slab or crack-isolation membrane	Most tile floors, bathrooms, kitchens
Wood Subfloor	Cement backer board or uncoupling membrane over plywood	Above-grade floors and wood joists
Existing Vinyl/Tile	Remove or use plywood + backer board or membrane	When existing surface is unstable or uneven
Wet Areas (Showers)	Waterproof membrane + cement board or foam board	Showers, wet rooms, steam rooms

Why Surface Preparation Matters

Tile is rigid and transfers loads to the substrate, so an unstable or wet base leads to failures. Proper layers under tile ensure structural support, moisture control, and adhesion. Skipping recommended layers often causes cracked tiles, loose grout, and delamination over time.

Assessing The Existing Subfloor

Begin by identifying the substrate: concrete slab, plywood over joists, or existing flooring. Check for levelness, deflection, moisture, and damage because each condition determines what to put down before tile and any repairs required.

Concrete Slab: What To Put Down

Concrete is the most straightforward base for tile but still needs prep. Clean, level, and check moisture content before applying thin-set mortar directly or installing a membrane for crack isolation or waterproofing.

Steps For Concrete Slab

• Clean Surface: Remove paint, oil, and debris to promote bonding.
• Moisture Test: Use a moisture meter or adhesive moisture test; high moisture requires a vapor barrier or waterproof membrane.
• Leveling: Use self-leveling compound for dips greater than 1/4″ in 10′.
• Crack Management: Apply crack-isolation membrane over hairline or static cracks.

Wood Subfloors: Recommended Layers

Wood subfloors require the most attention because they flex. Install a stable underlayment—cement backer board or an uncoupling membrane—over properly supported plywood to prevent movement-related tile failure.

Plywood Thickness And Joist Spacing

Use at least 3/4″ tongue-and-groove (T&G) plywood or 23/32″ OSB over joists spaced 16″ on center. Reduce deflection to L/360 or better for ceramic and porcelain tiles; natural stone may need L/720.

Cement Backer Board (CBU)

Cement backer boards like HardieBacker or Durock provide a rigid, cementitious layer. Fasten to subfloor with appropriate screws and fill joints with thin-set and tape before tiling for a stable base.

Uncoupling Membranes

Products such as Schluter-DITRA are polyethylene membranes that decouple tile from subfloor movement. They also provide waterproofing and vapor management and are particularly effective over plywood and radiant-heat installations.

Underlayment Options And When To Use Them

Choosing the right underlayment depends on substrate, moisture, and tile type. Main options include cement backer board, foam tile backer, uncoupling membranes, and crack-isolation membranes.

Underlayment	Benefits	Best For
Cement Backer Board	Rigid, water-resistant, cost-effective	Wood subfloors, baths, kitchens
Uncoupling Membrane	Decouples movement, waterproof options	Wood subfloors, radiant heat, larger format tile
Crack-Isolation Membrane	Stops slab cracks transmitting to tile	Concrete slabs with minor cracking
Foam Backer Board	Lightweight, waterproof, easy to cut	Showers, wet areas, prefabricated systems

Waterproofing: What To Put Down In Wet Areas

Wet areas require both a waterproof substrate and proper slope. Apply a bonded waterproof membrane over the substrate or use foam waterproof backer boards before tiling shower floors and walls to prevent moisture intrusion and mold.

Shower Pan And Wall Assembly

For showers, install a pre-sloped mortar pan or preformed pan, then a bonded waterproof membrane such as RedGard or Schluter-KERDI. Ensure seams and transitions are fully sealed and integrated with the shower drain.

Tile Adhesion: Mortars And Thin-Set

Thin-set mortar is the standard adhesive for ceramic and porcelain tile. Use modified thin-set for better bond strength, especially over cementitious backer boards and membranes. Choose a formulation rated for the tile size and substrate.

Choosing The Right Thin-Set

• Standard-modified thin-set: Good all-purpose choice.
• Large-format tile mortar: Higher bond strength and non-sag properties.

Tile Size And Substrate Requirements

Large-format tiles and stone require even more rigid substrates and flatter surfaces. Limit deflection, use uncoupling membranes, and ensure surface flatness within 1/8″ over 10′ or as manufacturer requires.

Dealing With Existing Flooring

Removing old vinyl or tile is often best, but in some cases it is acceptable to install tile over stable surfaces. Ensure the existing floor is rigid, clean, and well-bonded before adding underlayment or membrane.

When To Remove Existing Flooring

Remove floors that are loose, cushioned, or too thin to accept tile. Always remove vinyl that moves or has air gaps, and never tile over cracked tile without addressing the substrate.

Tools And Materials Needed For Prep

Essential tools include a moisture meter, level, trowels, mixing tools, cement board screws, backer board tape, thin-set, and a notched trowel. Specialty tools like a floor grinder or self-leveler may be required for concrete prep.

Step-By-Step Preparation Checklist

1. Identify substrate and check for damage or movement.
2. Test moisture levels on concrete and wood.
3. Repair or replace soft spots and reinforce joists if necessary.
4. Level surface with self-leveling compound if required.
5. Install chosen underlayment (backer board, membrane, or foam board).
6. Waterproof wet areas and seal transitions.
7. Choose and apply appropriate thin-set mortar for tile installation.

Common Mistakes To Avoid

Avoid installing tile over flexible or unstable surfaces, skipping waterproofing in wet areas, or using the wrong thin-set. Failing to address deflection, moisture, or levelness leads to early tile failure.

Cost Considerations And Labor

Costs vary by underlayment type: cement backer board is cost-effective but labor-intensive; uncoupling membranes can cost more but reduce risk and labor for crack isolation. Budget for removal, leveling, waterproofing, and underlayment when planning a tile project.

When To Hire A Professional

Hire a professional for complex substrates, significant moisture issues, irregular joist spacing, or when installing natural stone and large-format tiles. Professionals provide accurate flatness correction, proper waterproofing, and code-compliant installations.

Maintenance Tips After Installation

After tile is installed over the right layers, maintain grout seals and clean with pH-neutral cleaners. Inspect grout and caulked joints annually and reseal stone as required to extend tile life.

Additional Resources And Standards

Refer to industry standards such as The Tile Council Of North America (TCNA) installation methods and ANSI specifications for substrate preparation. Manufacturer recommendations trump general guidance when specified for products used.

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For specific product recommendations or a site-specific plan, consult local tile suppliers or a licensed installer to match materials to the job’s unique conditions. Correct preparation is the single most important step for durable, attractive tile installations.